1. Field of the Invention
The present invention relates to mechanical light shutter systems in which a motor drives a shutter wheel at a precise frequency; and more particularly, to the motor drive system for such a mechanical light shutter system used in an evoked potential autorefractometry system.
2. Description of the Related Art
Many scientific experiments are based on the response of an observed system to light. In these experiments, it is highly desirable that the system be exposed to light in synchronism with measurements of the system's response to the light. One example of such a scientific experiment is the reaction of a human being's eyes to light. In such a system, the brain waves that a person generates when viewing particular scenes can be low level signals; thus, the light source/motor drive system must generate minimum noise. Additionally, when detecting the brain waves of a person viewing a particular scene, it is necessary that the particular scene be alternated at a rate of less than 7 hertz so that the brainwaves indicating perception of the alternating scene are of a frequency less than the typical low limit for alpha waves. Typically, scenes are reversed at rate of 6 Hz.
FIG. 1 schematically illustrates prior means utilized by the applicant to generate the required reversal of a scene while simultaneously attempting to minimize the amount of noise generated by the motor drive system.
Referring to FIG. 1A, the applicant initally chose an AC motor 10 to drive a chopper wheel 15. The chopper wheel 15 can have printed thereon the particular pattern that is to be reversed and observed by a person. While the AC motor has very desirable noise characteristics in that the noise level is constant and can thus be subtracted from any measurements of the observed system. However, the A.C. motor can only operate at one frequency and can not track the frequency of an external signal source. The inability to track an external signal source is a major drawback because, when measuring a person's response to a pattern reversal, the human eye also picks up, for example, the flickering of fluorescent lamps which occurs at rate being a multiple of the 60 Hz power line frequency. It is desirable, therefore, to sychronize the reversal rate of the particular pattern and thus the persons perception of the pattern to an external source such as the 60 Hz line frequency so that the noise due to the overhead fluorescent lamps can be removed from the measured signal.
FIG. 1B schematically illustrates a second device for providing a pattern reversal rate developed by the applicant. This second approach utilizes a DC motor 20 and provides the ability to vary the frequency at which the chopper wheel 15 rotates. This approach, as with the first approach, has the drawback that it cannot track an external signal source. An additional drawback of this approach is that the DC motor generates correlated non-constant amplitude noise due to the brushes of the motor sliding over the slots in the commutator. Because the amplitude of this noise is nonconstant, brain wave measurements indicating the persons response to the pattern being reversed cannot be adjusted for this noise.
To provide the ability to track an external source, the applicant next developed the system schematically illustrated in FIG. 1C. This system employs a phase lock loop (PLL) controlled DC motor 25, and provides both variable frequency drive of the chopper wheel 15, and the ability to track an external signal source. This system, however, generates correlated, nonconstant amplitude noise due to the brushes crossing the slots in the commutator, and therefore as in the FIG. 1B system, this noise cannot be subtracted from the measured brain waves.
To eliminate the correlated noise, the applicant next utilized a stepper motor 30 as shown in FIG. 1D. This system has the beneficial features of a variable frequency drive for the chopper wheel 15 and constant amplitude noise that was correlated to the motion of the motor, enabling the brain wave measurements to be compensated for this noise. The system, however, could not track an external signal source and thus, could not be synchronized to an external noise source such as fluorescent lamps.